This invention relates to single-mode fiber optic couplers that are capable of effecting a relatively uniform coupling of light from one fiber to another over a relatively broad band of wavelengths.
Coupling occurs between two closely spaced cores in a multiple core device. Fiber optic couplers referred to herein as "fused fiber couplers" have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. The coupling efficiency increases with decreasing core separation and, in the case of single-mode cores, with decreasing core diameter.
European published patent application No. 0302745 teaches that various coupler properties can be improved by inserting the fibers into a capillary tube prior to heating and stretching the fibers, thereby resulting in the formation of an "overclad coupler". After the fibers have been inserted into the tube, the tube midregion is heated to cause it to collapse onto the fibers; the central portion of the midregion is thereafter drawn down to that diameter which is necessary to obtain the desired coupling. The coupling region of an overclad coupler is hermetically sealed, and the optical characteristics thereof are relatively insensitive to changes in temperature. The tube also greatly enhances the mechanical strength of the coupler.
Identical optical fibers are used to make overclad couplers referred to herein as "standard couplers", the coupling ratio of which is very wavelength dependent. A standard coupler which exhibits 3 dB coupling at 1310 nm cannot function as a 3 dB coupler at 1550 nm because of that wavelength dependence. A 3 dB coupler is one that couples 50% of the power from a first fiber to a second fiber. A standard coupler can be characterized in terms of its power transfer characteristics in a window centered about 1310 nm, which is referred to as the first window. For example, a standard coupler might exhibit a coupling ratio that does not vary more than about .+-.5% within a 60 nm window.
It has been known that an achromatic coupler, the coupling ratio of which is less sensitive to wavelength than it is for a standard coupler, can be formed by employing fibers having different propagation constants, i.e. by using fibers of different diameter and/or fibers of different refractive index profile or by tapering one of two identical fibers more than the other. There is no widely accepted definition of achromatic couplers. The least stringent definition would merely require an achromatic coupler to exhibit better power transfer characteristics than the standard coupler in the first window. More realistically, the specification is tightened by requiring an achromatic coupler to perform much better than the standard coupler in that first window, or to require it to exhibit low power transfer slopes in two windows of specified widths. These windows might be specified, for example, as being 100 nm wide and centered around about 1310 nm and 1530 nm. These windows need not have the same width; their widths could be 80 nm and 60 nm, for example. An optimally performing achromatic coupler would be capable of exhibiting low values of coupled power slope over essentially the entire single-mode operating region. For silica-based optical fibers this operating region might be specified as being between 1260 nm to 1580 nm, for example. It is noted that the total permissible variation in power includes insertion loss and that the permissible power variation specification becomes tighter as insertion loss increases. Furthermore, for a 3 dB coupler, for example, the coupled power at the center of the window should be 50%. If the 50% coupling wavelength is not at the center of the window, the coupled power specification becomes even tighter.
In the following discussion, the relative refractive index difference .DELTA..sub.a-b between two materials with refractive indices n.sub.a and n.sub.b is defined as EQU .DELTA..sub.a-b =(n.sub.a.sup.2 -n.sub.b.sup.2)/2n.sub.a.sup.2 ( 1)
For simplicity of expression, .DELTA. is often expressed in per cent, i.e. one hundred times .DELTA..
A usual requirement for fiber optic couplers is that the fibers extending therefrom, referred to herein as "pigtails", be optically and mechanically compatible with standard system fibers to which they will be connected in order to minimize connection loss. For example, the outside diameter and the mode field diameter of the coupler pigtails should be substantially the same as those of a standard fiber. One of the fibers employed in the fabrication of the coupler can be a standard, commercially available fiber. That feature of the other fiber that is modified to change the propagation constant should affect the outside diameter and mode field diameter of the pigtail portion of the other fiber as little as possible.
U.S. Pat. No. 4,798,436 (Mortimore) discloses a 3 dB fused fiber coupler wherein different propagation constants are obtained by pretapering one of the fibers. First and second identical standard fibers can be used to form such a coupler. The central portion of the first fiber is initially heated and stretched such that the core and the cladding diameter thereof in the tapered region is smaller than the core and cladding diameter of the second fiber. The pigtail portions of the stretched fiber can be connected with low loss to a standard system fiber since the ends thereof are identical to the ends of the stretched fiber. However, since a separate prestretching operation is employed for each coupler made, and since fiber diameter varies continuously along the length thereof, it is difficult to maintain process reproducibility. Also, a pretapered fiber is fragile and difficult to handle.
U.S. Pat. No. 4,822,126 (Sweeney et al.) teaches a 3 dB fused fiber coupler wherein .DELTA..sub.cores, the relative refractive index difference between the two coupler cores, is 0.061%. The value of .DELTA..sub.cores is obtained by substituting the two core refractive indices of the Sweeney et al. patent into equation (1) and solving for .DELTA.. It is apparent from FIG. 6 of the Sweeny et al. patent that the value of .DELTA..sub.cores should have been greater than 0.061% in order to have achieved good achromaticity with standard diameter fibers. However, when .DELTA..beta. is obtained by employing fibers having such large differences between the core refractive indices, the mode field diameter of one of the coupler pigtails differs sufficiently from that of a standard fiber that it will not efficiently couple to the fibers of the system in which the coupler is utilized. Rather than increasing the difference between the core refractive indices to provide a .DELTA..sub.cores greater than 0.061%, Sweeney et al. maintained that value of .DELTA..sub.cores and, in addition, etched the fiber claddings in order to improve achromaticity.
The Sweeny et al. patent states that although wavelength independence is achieved, as contemplated therein, by having the cores of different indices of refraction, similar results could be achieved by keeping the cores at like indices of refraction and making the claddings one different from the other with respect to indices of refraction. It will be obvious from the following discussion that it is impossible to form achromatic overclad-type 3 dB couplers wherein the difference between the refractive indices of the fiber claddings is such that .DELTA..sub.clads is 0.06%, assuming that the core and cladding diameters of the two fibers are identical. The value of .DELTA..sub.clads is obtained by substituting the cladding index n.sub.2 ' of one fiber and the cladding index n.sub.2 of the other fiber for n.sub.a and n.sub.b, respectively, of equation (1) and solving for .DELTA..